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Re: Come up with a brilliant solution .......



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It starts getting complicated because ;
   - LEDs can fail short-circuit OR open-circuit.
   - To get adequate luminance, the LED-lamp will probably consist
      of several parallel 'series-strings', where each 'series-string'
      would have several LEDs and a current-limiting resistor or
      driver connected in series fashion.
   - The environment is not electronic-friendly, what with noise,
      spikes and wide operating temperature-range.

Re: Come up with a brilliant solution .......


After thinking about the situation I am beginning to think the problem may be
more complex than it seems at first glance.

I presume the OP is talking about the sort of trailers that are attached to
'semi-trailer prime movers, and that the computer thingo is located in the
prime mover.  If this is so, then somehow that computer must have some way of
telling how many tailights/sidelights/number plate lights/brake
lights?/clearance lights etc are located on the trailer.  Do these vary from
trailer to trailer?  And remembering that one practice is for (say on the
Sydney to Melb run) there to be a change over of trailers at Tarcutta, then the
computer must be able to know how the lighting setup of any trailer that might
be connected to the prime mover.

Is any of this relevent to the OPs situation?

David

two bob wrote:

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Re: Come up with a brilliant solution .......


message
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Having just fired off a reply to your other post, this one deserves some
credit. That's a very valid point because either the controller in the rig
gets told how many lights on the trailer or it is assumed by the
manufacturer that there is a general response curve by a lamp, and that any
number (within some limit, no doubt) will behave in a certain fashion. The
LED's of course react differently. An intersting point is what happens if
only *some* of the lamps are replaced with LED's. At what point does the
system fail to detect the lights?

Ken


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Re: Come up with a brilliant solution .......


put finger to
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Whats your point? If there was a normal globe in cct, it would do the same
thing.



Re: Come up with a brilliant solution .......


On Sun, 25 Sep 2005 00:27:15 +1000, "two bob" <4> put finger to
keyboard and composed:

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My interpretation is that the transistor is DC coupled. Therefore, if
it is on whenever a pulse is present, then it would also be on
whenever the lights are on, so why use a transistor at all? The
proposal would only make sense if the base drive were AC coupled.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

Re: Come up with a brilliant solution .......


I was suggesting DC coupling, but thru a zener so the transistor only conducts
when the zener v is exceeded, by the pulse.

David

PS don't take my idea too seriously, as I may be way off track - just an idea

Franc Zabkar wrote:

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Re: Come up with a brilliant solution .......


seems to me all it would take is a capacitor in parallel with the lamp and
possibly a resistor to limit the current a bit...

my  guess is try 100-200 ufd and if it works add in a bit of resistance
until it quits working (use a 1k-10kpot) then back up till it works again
and measure the resistance...replace with a similar value fixed resistor and
your done.



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Re: Come up with a brilliant solution .......


finger to keyboard and composed:

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You need some way of drawing the required amount of current for the
duration of the pulse, and zero amps thereafter. What about a simple
circuit based around a parallel PTC resistor, ie something like what
happens inside a TV set during automatic degaussing at switch-on?

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

Re: Come up with a brilliant solution .......


Franc Zabkar furiously typed the following on 23/09/2005 8:22 AM:
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What about a feedback amp?

Re: Come up with a brilliant solution .......


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That would work for the pulse but would it work when the light was switched
on (as in tail lights - the PTC would go high resistance and the computer
would no longer detect enough current)? I think just a parallel resistor is
going to be required. Certainly the least complexity solution. Or maybe,
dare I say it, the light bulb.....

Ken



Re: Come up with a brilliant solution .......


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Surely the obvious solution is an electrolytic capacitor with large
valued parallel discharge resister and smaller series resister. When
the current pulse is sent, the series resister determines the initial
current draw. The parallel resister enables the cap to discharge for
another cycle.

How big a capacitor you need is determined by the required current
draw and the amount of time after switch-on that the current is sensed.
The idea will only be feasible of the time is very short. Normal
V=I*C*T rule applies.

Clifford Heath.

Re: Come up with a brilliant solution .......


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Or use the charge time of the cap to turn off a PNP power transistor which
would allow the use of very much smaller values.



Re: Come up with a brilliant solution .......


On Fri, 23 Sep 2005 10:04:18 +1000, Clifford Heath

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The LED array may itself provide an appropriate discharge time
constant.

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That should be dV = I * dT / C

or C = I * dT / dV

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-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

Re: Come up with a brilliant solution .......


how about, you make a circuit  that AC-couples (use a capacitor) the brake
line to a transistor that switches in a low impedance load (resistor) for a
short time (e.g. 50ms). each time the brake line pulses high, the pulse
turns on the transistor for a moment.

the constant "on" signal of the brakes won't trigger the circuit due to the
AC coupling.

such a short on-time won't effect brakelight performance any, but will be
long enough to allow the computer to 'sense' the higher load.






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globes



Re: Come up with a brilliant solution .......


finger to keyboard and composed:

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I had the same idea but declined to suggest it because the OP
specified a "simple" solution. I was thinking of something like this:


    |---------------------|
    |                  dummy R          R = 50 ohm ???
    |                     |
    |         Z          |/             Z = 6V zener
 o--|--C--|--|<|--|---R--| Q npn
    |     |       |      |\
  low R   D    hi R       |             D = diode (grounded anode)
    |_____|_______|__ ____|
                         _|_
                          =

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

Re: Come up with a brilliant solution .......



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How about

                          npn
                        C     E
  +V o---------------|---\___/-----> to to LED array
           |         |     |
          / E        |     |
       --| pnp       |--R1-|
      |   \ C        |
      R2   |         |
      |    RD        C1
      C2   |         |
      |    |         |
    o----------------|

RD emulates the incandescent-lamp load.
Select R2 & C2 time-constant to satisfy the duration
constaint of the 'lamp-test' and then to disconnect
that power-hungry load.

Select R1 & C1 time-constant to eliminate the
'light-test' pulses from flashing the leds at
night and dusk.

Seems that you'll need to determine two things ;
1. what the pulse-duration of the lamp-test
    actually is. (I suspect it'll be milliseconds
    if the pulses are visible during daylight)
and
2. what the 'lamp-ok' current threshold is.
    (10 mA, 100 mA, ???? )
    {A cold incandescent has a much lower resistance
     than a hot operating bulb.}


If one needs to verify that the led-strings are ok
then that will complicate things a lot because ;
1. the leds have such a fast response that the
    eye will detect the test-pulses at night/dusk,
and
2. a led in a series-string could be short OR open.

Re: Come up with a brilliant solution .......


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globes

Is the current only checked during the pulse?

Ross



Re: Come up with a brilliant solution .......


As far as we've been able to check this mess, yes, the current pulse is sent
when the ignition is switched on, and will keep on pulsing until the load
sense is satisfied. Vehicle diagnostics are performed (obviously, the lights
are only one of the tests) and any errors are indicated on the dashboard.
However - it would appear that the system has a continuous checking routine
to determine whether a lamp has failed during the run.
Sorry I sound so hazy over this, but trying to get a truck to play with is
not easy. Suppliers are not too interested, and the operators are not keen
on taking a truck out of service for us to investigate further.
Sometimes, I hate my job!!!!!


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Re: Come up with a brilliant solution .......



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How long is the pulse? Is is a consistant duration? I would go for a
somple circuit that switches in a load for a breaif period, perhaps
half a second, then switches off.


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Re: Come up with a brilliant solution .......



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I don't think there is any legitimate way around this, for the
following reason.

The whole idea of the computer check for blown lamps is to alert the
operator to a safety issue.

If you use ANY system which "fudges" the computer into thinking that
the led tail-lights are in fact incandescants (such as dummy loading,
transistor current loads etc) then what happens if somebody actually
disconnects the led lamps (try it out using a parallel incandescant
bulb)? With the dummy load in place the computer will think the led
tail lights are connected when in fact they are not. Such work-arounds
are potentially hazardous and should not be contemplated. Either the
manufacturer of the truck computer modifies his program to cater for
led lamps or you should stick to using incandescant bulbs.

Ross

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